The Gilder Lehrman Institute of American History in the article African Migration to Colonial America by author Ira Berlin explains, "The transatlantic slave trade had its beginning in the middle of the fifteenth century when Portuguese ships sailed down the West African coast. The intention was to trade for gold and spices, but the voyagers found another even more valuable commodity—human beings. Over time, the trade in men and women supplanted other commerce, and the slaves’ destination changed from Europe to the Americas, where plantations growing commodities for the international market initiated the massive transfer of African peoples. In all, some eleven to twelve million Africans were forcibly carried to the Americas. Of those, roughly one-half million (or about 4.5 percent) were taken to mainland North America or what became the United States."
I hope my research helped a bit :D
